Kanchanaburi is a province in west-central Thailand, bordering Myanmar along the Tenasserim range. In the gem trade the name refers principally to the Bo Phloi (sometimes Bo Ploi) sapphire field in the southern part of the province, which was an important source of basaltic blue sapphire to the Chanthaburi-Bangkok cutting industry from its discovery in the early 1980s through to the working out of the major shallow alluvial deposits in the 2000s.

Geological setting

The Bo Phloi sapphires are typical basalt-related corundum, formed under crustal conditions where iron-rich basaltic magmas brought corundum xenocrysts to the surface during late-Cenozoic volcanism. The same volcanic suite is responsible for the deposits at Pailin in adjacent western Cambodia and at the older Chanthaburi-Trat fields in eastern Thailand, and Bo Phloi material shares the same general gemmological signatures: high iron content, dark blue to inky body colour, often with a noticeable greenish component, and inclusions dominated by the small mineral crystals and silk patterns characteristic of basalt-host corundum.

Production history

Mining at Bo Phloi began informally in the late 1970s and intensified through the 1980s and 1990s, with both small-scale local operators and larger licensed concessions working the alluvial gravels. At its peak in the late 1980s and early 1990s Bo Phloi was supplying material in commercial quantities to Chanthaburi cutters, providing a domestic Thai source that complemented the larger flow of Australian and Sri Lankan sapphire rough being heated in Thai factories. Production declined through the 2000s as the most accessible alluvium was exhausted, and by the late 2010s commercial mining had effectively wound down, with sporadic small-scale activity continuing.

Material characteristics

Bo Phloi sapphires in their natural state tend to be too dark and inky for the international fine-stone market — a common feature of basalt-related corundum from this region. Heat treatment has been the standard remedy: low-iron-environment heating to lighten the body colour and reduce the green component, often with results that read as a serviceable medium-dark blue. The local industry has been one of the global centres for sapphire heating expertise, and Bo Phloi material was an important training ground for the techniques that the Bangkok trade later applied to East African and Madagascan rough.

Other Kanchanaburi gem occurrences

Beyond Bo Phloi, Kanchanaburi province has hosted minor production of black star sapphire, ruby, spinel and zircon from the same general basalt-related geological belt, and the Sangkhla Buri district near the Burmese border has yielded jadeite and tin-tungsten mineralisation that is not gemmologically significant. The principal contribution of the province to the trade remains the Bo Phloi sapphires of the late twentieth century.

Trade significance today

Kanchanaburi as an active sapphire origin is largely historical. The name appears in disclosure on older stones and in the literature on basalt-related sapphire formation, but contemporary Thai sapphire production from the province is small and locally consumed. The town of Bo Phloi retains a small cutting industry and a gem market, and parcels labelled Bo Phloi sometimes circulate at the Chanthaburi weekly trade, but the field is no longer a meaningful source for the international wholesale market. For graders attributing origin to a Thai sapphire, the principal task is distinguishing Bo Phloi material from the chemically similar Chanthaburi-Trat and Pailin sapphires, which is rarely possible with confidence on a per-stone basis.